Device for detecting the angular position of an object using a magnetoresistive sensor

ABSTRACT

A device for determining the angular position of an object uses an MR sensor which is set up in a magnetic field, where one magnetic field component (H or H g ) and a reference axis of the MR sensor can rotate relative to one another in a plane of rotation through an angle of rotation (Θ or φ) which has an unambiguous correlation to the angular position of the object; and the electric resistance of the MR sensor is an unambiguous function of this angle of rotation (Θ or φ). The MR sensor is designed with a giant MR layer system that contains at least one measurement layer with magnetization (M M ) that can be rotated through the magnetic field (H or H g ); the giant MR layer system also contains at least one biasing layer with a constant magnetization (M B ); and the resistance of the giant MR layer system is a function of the angle (α) between these two magnetizations (M m  and M B ).

BACKGROUND OF THE INVENTION

The present invention relates generally to devices for detecting theangular position of an object, and more particularly to a device fordetecting the angular position of an object relative to a preset zeroposition with a magnetoresistive (MR) sensor which has a constantreference axis plus contacts for supplying an electric current and isarranged in a magnetic field, where one magnetic field component (H orH_(g)) of the magnetic field and the reference axis of the MR sensor canbe rotated by an angle of rotation (Θ or φ) with respect to each otherin a plane of rotation, where this angle of rotation has an unambiguouscorrelation with the angular position to be determined, and theelectrical resistance of the MR sensor is an unambiguous function ofthis angle of rotation (Θ or φ). Such a device is disclosed, forexample, in the Philips Technical Information Brochure TI 901228"Properties and Applications of KMZ 10 Magnetic Field Sensors."

In one embodiment of this known device for measuring the angularposition, a magnetoresistive barber-pole sensor is set up in the fieldof a rotationally mounted magnet.

Magnetoresistive sensors are composed in general of a thin layer of amagnetoresistive material which is magnetized in the plane of the layer.When the magnetization of the layer is rotated with respect to thedirection of a measurement current flowing in the layer by a magneticfield which is to be measured, there is a change in resistance that mayamount to several percent of the normal isotropic resistance and can bedetected as a measurement signal. This effect is called anisotropicmagnetoresistance or the anisotropic magnetoresistive effect (AMR). Thecustomary magnetoresistive materials are ferromagnetic transition metalssuch as Nickel (Ni), iron (Fe), or Cobalt (Co) and alloys made withthese metals. At least one rectangular strip that is made of theferromagnetic NiFe alloy known commercially as Permalloy and ismagnetized in its longitudinal direction is provided with themagnetoresistive barber-pole sensor used in the known angle measurementdevice. Several thin metal strips are arranged side by side on thePermalloy strip at an angle of 45° to the longitudinal direction of thePermalloy strip. If voltage is now applied to the Permalloy strip in itslongitudinal direction, an electric current is generated between themetal strips, where the direction of this current is essentially at anangle of ±45° or ±135°, depending on the polarity of the voltage, to themagnetization of the Permalloy strip. An external magnetic field that isto be measured and has a component at right angles to the magnetizationthen rotates the magnetization of the Permalloy strip relative to thedirection of the current which remains constant. This rotation causes achange in resistance that has an approximately linear dependence on themagnetic field. The characteristic curve of the resistance of such abarber-pole sensor is unambiguous and at least approximately linear foran angle range of approximately 90°, which may be selected between about+45° to -45° or about -45° and +45° for the angle between themagnetization and the measurement current.

The magnetic field of the rotatable magnet is thus provided both as ameasurement field and as a supporting field to stabilize the sensorcharacteristic. Rotation of the magnet through an angle to be measuredcreates a change in the resistance signal of the barber-pole sensor. Themeasurement range of this angle-measuring device is limited to a maximumof ±90° because the sensor has an unambiguous characteristic curve onlyin this angular range. Larger angles up to ±135° can be achieved byusing the supporting field of an additional magnet. Angles of up toalmost ±180 degrees are feasible by using two sensors set up at rightangles to one another and analyzing their measurement signals in theindividual angle quadrants. Possible applications of this known deviceare for gas pedal sensors and throttle valve sensors for motor vehicles,gradient sensors, and wind direction indicators.

In another embodiment of this invention intended for use as a compass,two barber-pole sensors crossed at right angles are set up to rotate inthe magnetic field of a coil whose function is to reverse the magnetismof the sensors ("Technical Information Publication TI 901228" fromPhilips Components).

There are known multi-layered systems that have several ferromagneticlayers arranged in a stack and separated from one another byintermediate nonmagnetic metals layers, where the magnetization of eachmagnetic layer is in the plane of the layer. The thickness of each layeris selected so it is considerably smaller than the mean free path of theconduction electrons. When an electrical current is applied in such alayer system, the so-called magnetoresistive effect of giant magneticresistance (giant MR) also occurs in the individual layers in additionto the anisotropic magnetoresistive effect. This giant MR effect is dueto the varying intensity, as a function of the given magnetization, ofthe scattering of the majority and minority conduction electrons in thevolume of the layers, especially in alloys, but also at the interfacesbetween the ferromagnetic layers and the intermediate layers. This giantMR is an isotropic effect--in other words, it is not dependent on thedirection of the current in particular, and it can be considerablygreater than the anisotropic MR, with values of up to 70% of the normalisotropic resistance.

Two basic types of such giant MR multi-layered systems are known. In thefirst type, the ferromagnetic layers are linked togetherantiferromagnetically across the intermediate layers, so themagnetizations in the planes of two neighboring ferromagnetic layers arealigned antiparallel to each other without any external magnetic field.One example of this type would be the iron-chromium superlattices (Fe-Crsuperlattices) with ferromagnetic layers of Fe and antiferromagneticintermediate layers of Cr. With a properly aligned external magneticfield, the magnetization of the neighboring ferromagnetic layers isrotated against the antiferromagnetic coupling forces and is aligned inparallel. This reorientation of the magnetization by the magnetic fieldresults in a steady decrease in the giant MR, which is a measure of thesize of the magnetic field. At saturation field strength H_(s) there isno further change in the giant MR because all the magnetizations arethen aligned parallel to one another ("Physical Review Letters," vol.61, no. 21, Nov. 21, 1988, pp. 2472-2475).

In the second type of giant MR multi-layered system, the ferromagneticlayers are separated from one another by intermediate diamagnetic orparamagnetic layers made of metal. The thickness of the intermediatelayers is adjusted so the magnetic exchange coupling between themagnetizations of the ferromagnetic layers is as small as possible. Theneighboring ferromagnetic fields have different coercive fieldstrengths.

Using an exchange-decoupled giant MR layer system of this type withmagnetically softer measurement layers made of Ni₈₀ Fe₂₀ andmagnetically harder biasing layers made of Co separated from themeasurement layers by intermediate layers of Cu, the resistance hasalready been measured as a function of the angle Θ between a saturationmagnetic field strength H₀ and a magnetic field H_(r) rotating parallelto the planes of the layers. The size of the rotating field H_(r) was inthis case selected such that only the magnetization M₁ of themeasurement layers would follow the rotation of the magnetic fieldH_(r), and the magnetization M₂ of the biasing layers would persist inthe original alignment, which is determined by the saturation field H₀.It was found that the total electrical resistance R of the layer systemas a function of the angle Θ can be represented in good approximation bythe sum of a component A₁ ·cos (Θ) for the giant MR, a component A₂ ·cos(2·Θ) for the AMR, and a constant resistive component R₀, where thecoefficient A₂ of the AMR component is positive and the coefficient A₁of the giant MR component is negative and is about 60 times larger thanA₂ ("Journal of Magnetism and Magnetic Materials" (North-Holland), vol.113, 1992, pp. 79-82).

The present invention is directed to the problem of developing a devicefor measuring the angular position of an object as described in theintroduction so that an angular measurement of at least 180° isachieved, along with greater sensitivity and a larger field range.

SUMMARY OF THE INVENTION

The present invention solves this problem by providing that the MRsensor includes a system of layers stacked one on top of the other withthe planes of the layers at least approximately parallel to the plane ofrotation, and this layer system also has at least one measurement layerwith a magnetization (M_(M)) that can be rotated in the plane of thelayer through the magnetic field (H or H_(g)), and at least one biasinglayer with a magnetization (M_(B)) in the plane of the layer that ismostly independent of the magnetic field (H or H_(g)), and has anelectric resistance that is a function of the angle (α) between themagnetization (M_(M)) of the measurement layer and the magnetization(M_(B)) of biasing layer.

By using a giant MR sensor, it is possible to at least double themeasurement range because in contrast with the cos (2·Θ) dependence ofthe AMR, the giant MR has a cos(Θ) dependence on the angle of rotation Θbetween a magnetic field H rotating in the plane of the layer and aninitial magnetic field H₀. In addition, the sensitivity can be increasedby a factor of almost ten, and larger magnetic fields can be used.

In one embodiment according to the present invention, a change inangular position is converted to a rotation of a magnetic field H,whereas in another embodiment of the present invention, the change inangular position is converted to a corresponding rotation of themagnetoresistive sensor with respect to an external magnetic field H_(g)that is at least approximately constant. The first embodiment issuitable for automotive gas pedal sensors or throttle valve sensors, forexample, where angles of about 140° occur, and this embodiment ispreferably used for a noncontacting potentiometer. The second embodimentis preferably used as a compass.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic diagram of a device for measuring an angularposition with a rotating magnet.

FIG. 2 shows a schematic diagram of a device for measuring an angularposition with a giant MR sensor.

FIGS. 3 and 4 show schematic diagrams of a giant MR layer system for adevice according to the invention.

DETAILED DESCRIPTION

Parts which correspond to one another in these FIGS. are denoted by thesame reference numbers.

In the embodiment according to FIG. 1, an MR sensor 2 is provided with agiant MR layer system that is not shown in detail but is positioned inthe magnetic field of a magnet 4 that rotates around an axis of rotation6. Magnet 4 thus generates a magnetic field with a component H in aplane of rotation 5 normal to axis of rotation 6. This plane of rotation5 is aligned parallel to the planes of the individual layers of thegiant MR layer system. MR sensor 2 has a reference axis 3 in plane ofrotation 5, and an electric current I passes through it during themeasurement, preferably parallel to reference axis 3 and to the planesof the layers, but the current may also run in a direction normal to theplanes of the layers and to reference axis 3. The layer system containsat least one measurement layer with a magnetization M_(M) that can berotated through the magnetic field H and at least one biasing layer witha magnetization M_(B) that is at least approximately fixed (nonrotating)with respect to reference axis 3. MR sensor 2 with the layer system isdesigned as an elongated strip whose longitudinal direction correspondsto reference axis 3. The magnetization M_(B) of the biasing layer isalso parallel to the longitudinal direction of MR sensor 2. Thedemagnetizing fields can thus be attenuated.

In an especially advantageous embodiment of this invention that is notillustrated in a diagram, MR sensor 2 and the layers of its giant MRlayer system are designed to be at least approximately circular in orderto reduce problematical edge effects due to demagnetizing fields.

If the magnetic field H of magnet 4 in its starting position (denoted asinitial magnetic field H₀) is parallel to reference axis 3 of MR sensor2, then the magnetizations M_(M) of the measurement layer and M_(B) ofthe biasing layer are aligned parallel to one another and the angle αbetween them is 0°. Of course, the angle α can also be 180° if themagnetization M_(B) is antiparallel to the initial magnetic field H₀. Ifmagnet 4 is rotated around the axis of rotation 6, magnetic field Hrotates in plane of rotation 5 through a corresponding angle Θ withrespect to the initial magnetic field H₀.

The magnetization M_(M) of the measurement layer is also rotated at thesame time, resulting in a new angle a between the rotated magnetismM_(M) of the measurement layer and the fixed magnetism M_(B) of thebiasing layer. This angle α is in general approximately equal to theangle of rotation Θ or 180°+Θ. The electric resistance R of the giant MRlayer system through which the current I is flowing is an unambiguousfunction of the angle α between the two magnetizations M_(M) and M_(B)in an angle of 180°, and in fact the maximum resistance R is found atα=180° and the minimum is found at α=0°.

With this device, the angular position of a body in a plane parallel tothe plane of rotation 5 can be measured by converting the change inangular position into a rotation of magnet 4 about axis of rotation 6 bymeans of a shaft and then measuring the resistance R of MR sensor 2.

In an especially advantageous embodiment of the invention, the device isused for a noncontacting potentiometer.

Using a rotating device such as an adjusting knob, magnet 4 and itsmagnetic field H are rotated by means of a shaft and the resistance R ofthe layer system changes accordingly. Such a potentiometer has theadvantage that its electrically contacted part is not mechanicallycoupled to the rotating part. Therefore, additional partitions can beprovided between these two parts--namely, magnet 4 with the shaft androtating device on the one hand, and the layer system with itscorresponding electric contacts on the other hand--as long as thesepartitions do not reduce the stray field of magnet 4 too much.Furthermore, this potentiometer does not need a sliding contact, whichwould be subject to wear and corrosion.

Another advantageous application of the device according to thisinvention is for measuring and regulating the speed of electric motors.In this case, the axis of rotation 6 is preferably arranged parallel tothe carrying axle of the electric motor. Devices for measuring speed bymeasuring the angular position of the motor axle at regular timeintervals have now been provided. In comparison with known inductivepickups, this rotational speed measuring device has the advantage thateven low speeds can be determined, and the signal does not depend on thespeed.

Using a setup with two MR sensors whose reference axes are placed atright angles to each other, both the resting and moving angularpositions of the motor axle can be determined by squaring themeasurement signals of the two MR sensors.

FIG. 2 shows an embodiment of this invention with an MR sensor that canrotate about axis of rotation 6 and is placed in a spatially fixedmagnetic field H_(g). This device is especially suitable for use as acompass, with the earth's magnetic field serving as magnetic fieldH_(g). Rotation of reference axis 3 of MR sensor 2 through angle ofrotation φ from an initial position parallel to the magnetic field H_(g)again leads to a corresponding rotation of the magnetization M_(M) ofthe measurement layer. An angle α is formed between the magnetizationsM_(M) and M_(B) of the measurement layer and the biasing layer,respectively, and the resistance signal of the MR sensor 2 varies as afunction of this angle α.

FIGS. 3 and 4 illustrate embodiments of giant MR layer systems accordingto this invention.

FIG. 3 illustrates one embodiment of this invention with a measurementlayer 12 with a magnetization M_(M) in the plane of its layer and abiasing layer 16 with a fixed magnetization M_(B) in its plane.Measurement layer 12 and biasing layer 16 are magneticallyexchange-decoupled to a great extent by means of an electricallyconducting, nonmagnetic intermediate layer 14. An angle α resultsbetween the two magnetizations M_(M) and M_(B), and corresponds to theposition of the magnetic field H (not shown in the FIG). In thisembodiment of the invention, the magnetization M_(B) of biasing layer 16is stabilized by an additional magnetic layer 18 with a magnetizationM_(AF) that is antiparallel to the magnetization M_(B). This additionalmagnetic layer 18 is antiferromagnetically coupled to biasing layer 16on the side facing away from measurement layer 12.

In another embodiment of this invention (not shown in the FIGS.),biasing layer 16 is provided with a preferred axis and is magnetized inparallel to this preferred axis.

In another embodiment of this invention as illustrated in FIG. 4, anantiferromagnetic layer system is antiferromagnetically coupled tobiasing layer 16. This antiferromagnetic layer system consists of twomagnetic layers 18 and 22 whose magnetizations MAF1 are in the samedirection in the planes of the layers and a third magnetic layer 20 withmagnetization M_(AF2) antiparallel to MAF1, where each layer isantiferromagnetically coupled by means of intermediate layer 19 tomagnetic layers 18 and 22. With this arrangement of the layers, thedirection of magnetization M_(B) of biasing layer 16 can also be keptconstant.

The antiferromagnetic layer system can also be constructed from only twoantiferromagnetically coupled magnetic layers.

Intermediate layers are preferably provided between the magnetic layersof the antiferromagnetic layer system. In the embodiment of thisinvention illustrated in FIG. 3, an intermediate layer can also beplaced between biasing layer 16 and the antiferromagnetically coupledantiferromagnetic layer system or individual layer.

In a special embodiment of this invention, measurement layer 14 andbiasing layer 16 are antiferromagnetically or ferromagneticallyexchange-coupled to each other, preferably by means of an intermediatelayer, so their magnetizations M_(M) and M_(B) without the magneticfield H are aligned antiparallel or parallel to each other.

Preferably, several of the layer systems previously described with onemeasurement layer 12 and one biasing layer 16 are assembled in a stack,preferably a periodic arrangement. Typically, up to 100 such layersystems will be stacked one on top of the other.

For measurement purposes, the entire giant MR layer system is providedwith at least two contacts. These contacts are either both placed on thetop layer, so that on the whole the current I flows parallel to thelayer planes (current-in-plane=cip); or as an alternative, one contactis placed on the top layer and one is placed on the bottom layer, so thecurrent I flows mostly at right angles to the layer planes(current-perpendicular-to-planes=cpp).

What is claimed is:
 1. In a device for detecting an angular position ofan object relative to a preset zero position using a magnetoresistive(MR) sensor which has a constant reference axis plus contacts forsupplying an electric current and is arranged in a magnetic field, whereone magnetic field component of the magnetic field and reference axis ofthe MR sensor can be rotated by an angle of rotation with respect toeach other in a plane of rotation, where the angle of rotation has anunambiguous correlation with the angular position to be determined, andthe electrical resistance of the MR sensor is an unambiguous function ofthe angle of rotation, an MR sensor comprising:a) a system of layersstacked one on top of another with a plane of each of the layers beingat least approximately parallel to the plane of rotation, said layersystem including:(i) at least one measurement layer having amagnetization (M_(M)) that can be rotated in a plane of the measurementlayer through the magnetic field; (ii) at least one biasing layer havinga magnetization (M_(B)) in a plane of the biasing layer that is mostlyindependent of the magnetic field, wherein the layer system has anelectric resistance that is a function of an angle between themagnetization (M_(M)) of the measurement layer and the magnetization(M_(B)) of the at least one biasing layer; and (iii) at least onemagnetic layer being disposed on a side of said at least one biasinglayer that faces away from the measurement layer, the at least onemagnetic layer having a magnetization (M_(AF)) antiparallel to themagnetization (M_(B)) of the at least one biasing layer and stabilizingthe magnetization (M_(B)) of the at least one biasing layer byantiferromagnetically coupling to the at least one biasing layer.
 2. TheMR sensor according claim 1, wherein the system of layers furthercomprises an intermediate layer disposed between the measurement layerand the biasing layer, whereby the measurement layer is at leastapproximately magnetically exchange-decoupled from the biasing layer bythe intermediate layer.
 3. The MR sensor according claim 1, wherein thebiasing layer has a coercive field strength that is quantitativelygreater than the magnetic field, and the measurement layer has acoercive field strength that is quantitatively smaller than the magneticfield.
 4. The MR sensor according to claim 1, wherein the system oflayers further comprises an intermediate layer disposed between themeasurement layer and the biasing layer, whereby the measurement layerand the biasing layer are exchanged-coupled by the intermediate layer.5. The MR sensor according to claim 1, wherein said biasing layer isdirectly joined to the magnetic layer.
 6. The MR sensor according toclaim 1, wherein the system of layers further comprises an intermediatelayer disposed between the biasing layer and the magnetic layer.
 7. TheMR sensor according to claim 1, further comprising at least one secondmagnetic layer coupled to the at least one magnetic layer.
 8. The MRsensor according to claim 7, further comprising an intermediate layerdisposed between the at least one magnetic layer and the at least onesecond magnetic layer.
 9. The MR sensor according to claim 1, whereinall layers in the system of layers are arranged in a stack in a periodicsequence.
 10. The MR sensor according to claim 1, wherein the layersystem is at least approximately circular in shape.
 11. A device fordetecting an angular position of an object relative to a preset zeroposition, comprising a magnetoresistive (MR) sensor having:a) a constantreference axis; b) a plurality of contacts for supplying an electriccurrent, wherein the MR sensor is disposed in a magnetic field, onemagnetic field component of the magnetic field and the reference axis ofthe MR sensor can be rotated by an angle of rotation with respect toeach other in a plane of rotation, the angle of rotation has anunambiguous correlation with the angular position to be determined, andan electrical resistance of the MR sensor is an unambiguous function ofthe angle of rotation; c) a system of layers stacked one on top ofanother with a plane of each of the layers being at least approximatelyparallel to the plane of rotation, said layer system including:(i) atleast one measurement layer having a magnetization (M_(M)) that can berotated in a plane of the measurement layer through the magnetic field;(ii) at least one biasing layer having a magnetization (M_(B)) in aplane of the biasing layer that is mostly independent of the magneticfield, wherein the layer system has an electric resistance that is afunction of an angle between the magnetization (M_(M)) of themeasurement layer and the magnetization (M_(B)) of the at least onebiasing layer; and (iii) at least one magnetic layer being disposed on aside of said at least one biasing layer that faces away from themeasurement layer, the at least one magnetic layer having amagnetization (M_(AF)) antiparallel to the magnetization (M_(B)) of theat least one biasing layer and stabilizing the magnetization (M_(B)) ofthe at least one biasing layer by antiferromagnetically coupling to theat least one biasing layer.
 12. The device according to claim 11,wherein the MR sensor is disposed in a fixed location spatially and aplurality of devices are provided for generating a magnetic field thatrotates with respect to the reference axis of the MR sensor in the planeof rotation.
 13. The device according to claim 11, wherein the device isused as a noncontacting potentiometer.
 14. The device according to claim11; wherein the magnetic field is in a fixed location and the referenceaxis of the MR sensor can be rotated in the plane of rotation.
 15. Thedevice according to claim 11, wherein the system of layers furthercomprises an intermediate layer disposed between the measurement layerand the biasing layer, whereby the measurement layer is at leastapproximately magnetically exchange-decoupled from the biasing layer bythe intermediate layer.
 16. The device sensor according to claims 11;wherein the system of layers further comprises an intermediate layerdisposed between the measurement layer and the biasing layer, wherebythe measurement layer and the biasing layer are exchanged-coupled by theintermediate layer.
 17. The device according to claim 11, wherein thebiasing layer has a preferred magnetic axis and is magnetized along thepreferred axis.
 18. The device according to claim 11, wherein saidbiasing layer is directly joined to the magnetic layer.
 19. The device1according to claim 11, wherein the system of layers further comprisesan intermediate layer disposed between the biasing layer and themagnetic layer.
 20. The system according to claim 11, further comprisingat least one second magnetic layer coupled to the at least one magneticlayer.
 21. The system according to claim 20, further comprising anintermediate layer disposed between the at least one magnetic layer andthe at least one second magnetic layer.
 22. The device according toclaim 20, further comprising a plurality of intermediate layers at leastpartially provided between the magnetic layers.
 23. The device accordingto claim 11, wherein the layers in the system of layers are arranged ina stack in a periodic sequence.
 24. The device according to claim 11,wherein the layer system is at least approximately circular in shape.25. The device according to claim 11, further comprising an additionalMR sensor having its reference axis aligned normal to the reference axisof the other MR sensor.
 26. An apparatus for detecting rotational motionof an object by detecting an angular position of the object relative toa preset zero position at regular intervals, including a plurality ofdevices, each device comprising a magnetoresistive (MR) sensor having:a)a constant reference axis; b) a plurality of contacts for supplying anelectric current, wherein the MR sensor is disposed in a magnetic field,one magnetic field component of the magnetic field and the referenceaxis of the MR sensor can be rotated by an angle of rotation withrespect to each other in a plane of rotation, the angle of rotation hasan unambiguous correlation with the angular position to be determined,and an electrical resistance of the MR sensor is an unambiguous functionof the angle of rotation; c) a system of layers stacked one on top ofanother with a plane of each of the layers being at least approximatelyparallel to the plane of rotation, said layer system including:(i) atleast one measurement layer having a magnetization (M_(M)) that can berotated in a plane of the measurement layer through the magnetic field;(ii) at least one biasing layer having a magnetization (M_(B)) in aplane of the biasing layer that is mostly independent of the magneticfield, wherein the layer system has an electric resistance that is afunction of an angle between the magnetization (M_(M)) of themeasurement layer and the magnetization (M_(B)) of the at least onebiasing layer; and (iii) at least one magnetic layer being disposed on aside of said at least one biasing layer that faces away from themeasurement layer, the at least one magnetic layer having amagnetization (M_(AF)) antiparallel to the magnetization (M_(B)) of theat least one biasing layer and stabilizing the magnetization (M_(B)) ofthe at least one biasing layer by antiferromagnetically coupling to theat least one biasing layer.